Inertial sensors are used in various applications, such as, for example, in the automobile field to control various devices, such as air bags, anti-lock braking systems (ABS), active suspensions, or in other fields, such as consumer electronics, electronic processors and the like. Inertial sensors are operable to measure a mechanical quantity, for example, related to a movement of the sensor (such as its acceleration) and to produce an output signal as a function of this quantity. One particular type of inertial sensor, which is extremely small and very reliable, is that made on a microscopic scale (micro sensors) on a chip of semiconductor material. Inertial sensors of this type are based on the displacement of a sensing element, movable with respect to the chip, the movement of which is converted into a suitable electrical signal. Preferably, a circuit for processing the output signal produced by the sensor should also be integrated onto this chip.
In semiconductor inertial sensors the sensitive element must be suitably isolated by enclosing it in a sealed structure in such a way as to ensure that it operates in a controlled environment. This allows the sensitive element, which has a very small mass, to move with low resistance and minimum damping to guarantee good sensitivity of the sensor. Moreover, the circuit for processing the output signal from the sensor requires encapsulation in a suitable container or package which protects the processing circuit from the external environmental conditions thereby guaranteeing its correct operation.
One method for hermetically enclosing a semiconductor inertial sensor includes encapsulating it with the associated circuitry in a sealed container, such as, for example, a ceramic or metal container. This technique, however, is extremely expensive, which converts into a high cost of the finished product because the cost of the container represents the major proportion of the overall costs.
A different known technique includes producing a hollow structure on the microscopic scale (micro cavity) to house the sensing element. This isolating method involves the micro-working of a wafer of silicon or glass which is then connected to the wafer on which the inertial sensors are formed, for example, by means of an anodic bonding technique. This method makes it possible to use standard plastic containers of low cost for encapsulating the final product. The above described known arrangement is nevertheless rather expensive and has the disadvantage of not allowing the sensor and the associated processing circuitry to be integrated onto the same chip so that the connection step requires that the surfaces to be joined are perfectly planar (with a peak-to-trough roughness of the order of tens of .ANG.)